U.S. patent number 5,499,268 [Application Number 08/404,842] was granted by the patent office on 1996-03-12 for adaptive equalizer capable of compensating for carrier frequency offset.
This patent grant is currently assigned to Japan Radio Co., Ltd.. Invention is credited to Kyo Takahashi.
United States Patent |
5,499,268 |
Takahashi |
March 12, 1996 |
Adaptive equalizer capable of compensating for carrier frequency
offset
Abstract
An adaptive equalizer includes a multiplier for multiplying, by
corrective data, an output signal from a filter unit for
compensating for a signal distortion to which input digital data
are subjected, a decision unit for estimating and outputting
symbols of output data from the multiplier, a subtractor for
subtracting an output signal of the decision unit from the output
data from the multiplier, multipliers for inversely correcting the
output signals from the decision unit and the subtractor which are
corrected by the multiplier, a coefficient updating unit for
updating the coefficients of the filter unit based on an output
signal from the multiplier which inversely corrects the output
signal from the subtractor, and a frequency offset estimating unit
for estimating corrective data based on a frequency offset on the
basis of the output signal from the multiplier which inversely
corrects the output signal from the subtractor, and using the
estimated corrective data as corrective data for the multiplier
which multiplies the output signal from the filter unit by
corrective data. An output signal from the multiplier which
inversely corrects the output signal from the decision unit is fed
back to a feedback filter of the filter unit.
Inventors: |
Takahashi; Kyo (Mitaka,
JP) |
Assignee: |
Japan Radio Co., Ltd. (Tokyo,
JP)
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Family
ID: |
27340344 |
Appl.
No.: |
08/404,842 |
Filed: |
March 20, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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162339 |
Dec 3, 1993 |
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Foreign Application Priority Data
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Dec 9, 1992 [JP] |
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4-329245 |
Dec 15, 1992 [JP] |
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4-334501 |
Dec 17, 1992 [JP] |
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4-337587 |
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Current U.S.
Class: |
375/231; 329/307;
375/232; 375/233; 375/324; 375/350; 708/323 |
Current CPC
Class: |
H04L
25/03019 (20130101); H04L 27/2332 (20130101); H04L
2027/003 (20130101); H04L 2027/004 (20130101); H04L
2027/0053 (20130101); H04L 2027/0095 (20130101) |
Current International
Class: |
H04L
25/03 (20060101); H04L 27/233 (20060101); H04L
27/00 (20060101); H03H 007/30 (); H03H 007/40 ();
H03K 005/159 () |
Field of
Search: |
;375/229-233,235,266,324,331,344,350 ;364/724.19,724.2
;329/307,325,360 ;333/28R ;455/337 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2067461 |
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Dec 1992 |
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CA |
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0369406 |
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May 1990 |
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EP |
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Other References
NEC Research and Development, No. 45, Apr. 1977, Tokyo, Japan, pp.
38-49, Akashi et al., "A High Performance Digital QAM 9600 bit/s
Modem". .
Shahid U. H. Qureshi, Adaptive Equalization, Sep. 1985, pp.
1349-1387, Proceedings of the IEEE, vol. 73, No. 9, New York,
U.S.A..
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Primary Examiner: Chin; Stephen
Assistant Examiner: Le; Amanda T.
Attorney, Agent or Firm: Frishauf, Holtz, Goodman, Langer
& Chick
Parent Case Text
This application is a Continuation of application Ser. No.
08/162,339, filed Dec. 3, 1993.
Claims
What is claimed is:
1. An adaptive equalizer comprising:
a filter for compensating for a transmission distortion to which an
input signal is subjected, wherein the input signal comprises a
training signal and data following the training signal;
decision means for estimating and outputting symbols of the input
signal in response to an output signal from said filter;
training signal generating means for outputting a training
signal;
a first selector for selecting one of an output signal from said
decision means and the training signal outputted from said training
signal generating means;
coefficient updating means for controlling coefficients of said
filter based on an output signal which is produced by subtracting
an output signal of said first selector from the output signal of
said filter;
a memory for storing the input signal;
a modulator for successively modulating signals, as a carrier,
having a plurality of frequencies within a predetermined frequency
range containing an expected frequency offset with the training
signal read from said memory;
a multiplier for multiplying the input signal read from said memory
by a corrective signal;
a second selector for selectively supplying one of output signals
from said multiplier and said modulator to said filter;
an adder for adding, while the input signal is the training signal,
an output signal, per carrier frequency, which is produced by
subtracting the output signal of said filter supplied with a
modulated training signal outputted through said modulator and said
second selector, from the training signal outputted through said
first selector; and
correcting means for converting the carrier frequency corresponding
to a minimum value of a sum output signal from said adder into said
corrective signal based on said carrier frequency while the input
signal is the training signal, and for supplying said corrective
signal to said multiplier while the input signal is the data.
2. An adaptive equalizer according to claim 1, wherein said
corrective signal is a unit vector having a phase angle based on
the carrier frequency corresponding to the minimum value of the sum
output signal from said adder, and said multiplier is a complex
multiplier for multiplying the input signal read from said memory
by said unit vector.
3. An adaptive equalizer according to claim 1, wherein said
correcting means comprises:
converting means for converting the carrier frequency corresponding
to a minimum value of a sum output signal from said adder into a
voltage based on said carrier frequency; and
a local oscillator in a stage preceding the adaptive equalizer for
controlling an oscillating frequency in response to the voltage
which is supplied as a corrective control voltage from said
converting means.
4. An adaptive equalizer comprising:
a memory for storing an input signal comprising a training signal
and data following the training signal;
a modulator for successively modulating a plurality of frequencies,
as a carrier, within a predetermined frequency range containing an
expected frequency offset with the training signal read from said
memory;
a filter for compensating for a transmission distortion to which
the input signal is subjected;
a first selector for selecting and supplying one of an output
signal from said modulator and stored contents of said memory to
said filter;
correcting means for correcting an output signal from said filter
with corrective data;
decision means for estimating and outputting symbols of the input
signal in response to an output signal from said correcting
means;
training signal generating means for outputting a training
signal;
a second selector for selecting one of an output signal from said
decision means and the training signal outputted from said training
signal generating means;
inversely correcting means for inversely correcting a difference
output signal produced by subtracting an output signal of said
second selector from the output signal of said correcting means and
corrected by said correcting means;
coefficient updating means for updating coefficients of said filter
based on an output signal from said inversely correcting means;
initial value estimating means for adding, per carrier frequency,
the output signal from said inversely correcting means when the
output signal from said modulator is selected by said first
selector and the training signal outputted from said training
signal generating means is selected by said second selector, and
for converting a carrier frequency corresponding to a minimum value
of a sum into corrective data as an estimated initial value based
on said carrier frequency; and
frequency offset estimating means for estimating corrective data
based on the output signal from said inversely correcting means,
using the estimated initial value from said initial value
estimating means as an initial corrective value, when the stored
contents of said memory are selected by said first selector and the
output signal from said decision means is selected by said second
selector, and for outputting the estimated corrective data to said
correcting means.
5. An adaptive equalizer according to claim 4, wherein said
inversely correcting means comprises:
complex conjugate converting means for converting the corrective
data from said frequency offset estimating means into complex
conjugate data; and
a complex multiplier for multiplying output data from said complex
conjugate converting means by said difference output signal.
6. An adaptive equalizer according to claim 4, wherein said
frequency offset estimating means comprises:
converting means for converting the output signal from said
inversely correcting means into a phase quantity based on the
output signal from said inversely correcting means;
first accumulating/adding means for accumulating and adding phase
quantities outputted from said converting means for a predetermined
period;
second accumulating/adding means for accumulating and adding output
phase quantities from said first accumulating/adding means for a
predetermined period; and
a vector data converter for converting an output phase quantity
from said second accumulating/adding means into a unit vector, as
the corrective data, which has a phase angle based on the output
phase quantity from said second accumulating/adding means.
7. An adaptive equalizer comprising:
a memory for storing an input signal comprising a training signal
and data following the training signal;
a modulator for successively modulating a plurality of frequencies,
as a carrier, within a predetermined frequency range containing an
expected frequency offset with the training signal read from said
memory;
a filter for compensating for a transmission distortion to which
the input signal is subjected;
a first selector for selecting and supplying one of an output
signal from said modulator and stored contents of said memory to
said filter;
correcting means for correcting an output signal from said filter
with corrective data;
decision means for estimating and outputting symbols of the input
signal in response to an output signal from said correcting
means;
training signal generating means for outputting a training
signal;
a second selector for selecting one of an output signal from said
decision means and the training signal outputted from said training
signal generating means;
first inversely correcting means for inversely correcting an output
signal from said second selector which is corrected by said
correcting means, and feeding back an inversely corrected output
signal to said filter;
second inversely correcting means for inversely correcting a
difference output signal produced by subtracting an output signal
of said second selector from the output signal of said correcting
means and corrected by said correcting means;
coefficient updating means for updating coefficients of said filter
based on an output signal from said second inversely correcting
means;
initial value estimating means for adding, per carrier frequency,
the output signal from said second inversely correcting means when
the output signal from said modulator is selected by said first
selector and the training signal outputted from said training
signal generating means is selected by said second selector, and
for converting a carrier frequency corresponding to a minimum value
of a sum into corrective data as an estimated initial value based
on said carrier frequency; and
frequency offset estimating means for estimating corrective data
based on the output signal from said second inversely correcting
means, using the estimated initial value from said initial value
estimating means as an initial corrective value, when the stored
contents of said memory are selected by said first selector and the
output signal from said decision means is selected by said second
selector, and for outputting the estimated corrective data to said
correcting means.
8. An adaptive equalizer according to claim 7, wherein:
said first inversely correcting means comprises:
complex conjugate converting means for converting the corrective
data from said frequency offset estimating means into complex
conjugate data; and
a first complex multiplier for multiplying output data from said
complex conjugate converting means by the output signal from said
second selector; and
said second inversely correcting means comprises a second complex
multiplier for multiplying the output data from said complex
conjugate converting means by said difference output signal.
9. An adaptive equalizer according to claim 7, wherein said
frequency offset estimating means comprises:
converting means for converting the output signal from said second
inversely correcting means into a phase quantity based on the
output signal from said second inversely correcting means;
first accumulating/adding means for accumulating and adding phase
quantities outputted from said converting means for a predetermined
period;
second accumulating/adding means for accumulating and adding output
phase quantities from said first accumulating/adding means for a
predetermined period; and
a vector data converter for converting an output phase quantity
from said second accumulating/adding means into a unit vector, as
the corrective data, which has a phase angle based on the output
phase quantity from said second accumulating/adding means.
10. An adaptive equalizer according to claim 4 or 7, wherein said
correcting means comprises a complex multiplier for multiplying the
output signal from said filter by said corrective data from said
frequency offset estimating means.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an adaptive equalizer for use in
digital cellular receiver terminals or the like, and more
particularly to an adaptive equalizer capable of compensating for a
deterioration in the channel characteristics which results from a
carrier frequency offset.
2. Description of the Related Art
Heretofore, land mobile communications systems, typically
automobile telephone system, have been analog communications
systems. To meet demands for a rapid increase in the number of
subscribers to such land mobile communications systems, more
diverse types of data to be transmitted, and compatibility with
ISDN (Integrated-Services Digital Network), efforts are being made
to develop digital mobile communications systems.
For example, the Telecommunication Industries Association in the
U.S.A. established in 1989 the digital automobile telephone
standards which are summarized as follows:
Frequency band: 800/900 MHz
Access method: TDMA
Audio encoding method: 13 kbps VSELP
Number of channels per wave: 3
Carrier interval: 60 kHz (30 kHz interleave)
Modulation method: .pi./4-shift DQPSK
Base station radius: 0.5.about.20 km
A .pi./4-shift DQPSK (.pi./4-shift differentially encoded
quadrature phase shift keying) signal is a signal which is produced
by differentially encoding symbols to be transmitted and then
subjecting them to .pi./4-shift QPSK.
The process of .pi./4-shift QPSK will be described below. The
differentially encoding process that has no effect on an
understanding of the present invention will not be described
below.
In .pi./4-shift QPSK, a series of bits of digital signals 0 and 1
is divided into pairs of bits, and the phase angle .theta..sub.k of
a high-frequency sine wave is determined depending on one of 2-bit
combinations 00, 01, 10, 11 (a 2-bit combination {X.sub.k, Y.sub.k
} is referred to as a "symbol"). A sine wave S(t) having a phase
angle .theta..sub.k corresponding to the kth symbol is expressed
by:
where
.omega..sub.c is the angular frequency of a carrier sine wave (if a
carrier frequency is f.sub.c, then .omega..sub.c =2.pi.f.sub.c),
and
T is the duration of one symbol.
The sine wave S(t) may also be expressed as follows:
where
The values of (a.sub.k, b.sub.k) represent the components of a
symbol on an I-Q rectangular Cartesian coordinate plane composed of
an in-phase axis (I axis) and a quadrature axis (Q axis).
FIG. 8 of the accompanying drawings shows, by way of example, a
conventional digital cellular receiver for receiving QPSK-modulated
waves. It is assumed that the digital cellular receiver has
received a signal R(t) that is expressed by:
where (a', b') is (a.sub.k, b.sub.k) that has been received (the
suffix k is omitted).
In the digital cellular receiver shown in FIG. 8, the received
signal R(t) expressed by the equation (6) is subjected to
quadrature detection to reproduce the combinations (a.sub.k,
b.sub.k) (and further to determine phase differences between
succeeding combinations (a.sub.k, b.sub.k) in differential
decoding) thereby reproducing the symbols, and then demodulate the
symbols into a series of bits 0 and 1 which is original serial
signals.
The quadrature detector divides the received signal expressed by
the equation (6) into two signals, multiplies one of the signal by
a sine wave cos(.omega..sub.c t) which is of the same frequency and
phase as the transmitted carrier, and multiplies the other signal
by a sine wave sin(.omega..sub.c t). This quadrature detection
process is called a synchronous detection process. The results of
the process are given as follows:
and
The signals expressed by the above equations (7) and (8) are passed
through a low-pass filter to remove multiple frequency components
therefrom, thus obtaining (1/2)a', (1/2)b'.
In the above synchronous detection process, however, it is
necessary to generate a carrier whose frequency and phase are equal
to those of the transmitted carrier. Methods of extracting and
reproducing such a carrier in a receiver generally include inverse
modulation, multiplication, and Costas loop. These methods
reproduce a carrier based on waveform information contained in the
received signal. Therefore, if the received signal has a distorted
waveform due to multipath fading, for example, then they fail to
extract and reproduce a carrier with high accuracy. Under such an
adverse condition, the synchronous detection process cannot be
relied upon.
In conventional digital communications between stationary stations,
there has been employed an adaptive equalizer to compensate for a
decoding error rate because they are also susceptible to multipath
fading. FIG. 9 of the accompanying drawings illustrates, for
example, an adaptive equalizer in the digital communication
terminal shown in FIG. 8.
An output signal (a', b') from the synchronous detector is inputted
to a demultiplexer which selects a signal of its own slot and sends
it to the adaptive equalizer.
As shown in FIG. 9, the adaptive equalizer comprises a filter unit
composed of a feed-forward filter and a feedback filter for
processing a complex input signal whose real part is the I
component of the output signal from the synchronous detector and
imaginary part is the Q component of the output signal from the
synchronous detector, the feed-forward and feedback filters having
complex coefficients, a decision unit for determining the phase of
an output signal from the filter unit, a complex adder for
calculating an equalization error signal, a coefficient updating
unit for updating the coefficients of the feed-forward and feedback
filters based on the equalization error signal according to an
algorithm, and a training signal generator for training the
adaptive equalizer.
The input signal (a', b') is filtered by the filter unit to remove
a waveform distortion due to multipath fading therefrom, and then
sent to the decision unit. If it is assumed that the filter unit
outputs a signal (a.sub.of, b.sub.of), then the decision unit
determines which phase of the equation (4) the output signal from
the filter unit corresponds to, and outputs a signal (a.sub.dec,
b.sub.dec) corresponding to the phase. The complex adder determines
the difference (a.sub.of -a.sub.dec, b.sub.of -b.sub.dec) between
the output signal (a.sub.of, b.sub.of) from the filter unit and the
output signal (a.sub.dec, b.sub.dec) from the decision unit, and
outputs the difference as an equalization error signal. The
coefficient updating unit updates the coefficients of the
feed-forward and feedback filters. The output signal (a.sub.dec,
b.sub.dec) from the decision unit is fed back to the feedback
filter. The adaptive equalizer of this type is referred to as a
decision feedback equalizer, which is known to be effective in
compensating for a delay dispersion of a received signal due to
multipath fading.
Digital mobile communication devices are more susceptible to
multipath fading than conventional digital communication devices
for use between stationary stations because they are often required
to communicate with each other in locations such as between
buildings or the like in cities. Therefore, the receivers of
digital mobile communication terminals should be equipped with an
oscillator for generating a detecting carrier to carry out
detection (quasi-synchronous detection) similar to the synchronous
detection using the oscillated detecting carrier.
Since the frequency of the transmitted carrier is known, the
oscillator in the receiver is required to generate a carrier having
the same frequency as the frequency of the transmitted carrier.
However, such a requirement may not necessarily be met. It is also
impossible to eliminate the phase difference. In the
quasi-synchronous detection, therefore, it is necessary to effect
quadrature detection using the detecting carrier whose frequency
and phase are slightly different from those of the transmitted
carrier, for reproducing a transmitted series of symbols.
The quadrature detector for carrying out the quasi-synchronous
detection divides the received signal expressed according to the
equation (6) into two signals, multiplies one of the signals by a
sine wave cos(.omega.'t+.theta.), and multiplies the other signal
by a sine wave sin(.omega.'t+.theta.), where .omega.' is the
angular frequency of the detecting carrier which is different from
the frequency of the transmitted carrier, and .theta. the phase
difference between the detecting carrier and the transmitted
carrier. The signals produced by the above multiplication are
passed through a low-pass filter, which outputs the following
signals:
where .DELTA..omega. is the difference between the transmitted
carrier .omega..sub.c and the detecting carrier .omega.', and
called a carrier offset.
As can be seen from the equations (9) and (10), the signal (a', b')
produced as a result of the quasi-synchronous detection is
expressed as a vector, on the I-Q plane, whose absolute value is
(1/2)(a'.sup.2 +b'.sup.2).sup.1/2 and which keeps rotating at an
angular velocity .DELTA..omega.. While the vector (a', b') is
rotating, if the angular velocity .DELTA..omega. exceeds about 10
Hz, then the error rate is large with the normal decoding process.
Therefore, it is necessary to detect and compensate for a carrier
offset with some means.
The manner in which the adaptive equalizer responds to a carrier
offset will be described below.
If an input signal produced by quadrature detection of a signal
which is received by the receiver and applied to the adaptive
equalizer contains a carrier offset .DELTA..omega., then the
spectrum Reql(.omega.) of the input signal is represented by:
where W(.omega.) is the spectrum of a transmitted series of symbols
w.sub.i, H(.omega.) the spectrum of an impulse response h(t) of the
transmission path, and G(.omega.) the spectrum of an impulse
response g(g) of the waveform shaping filter. These spectrums are
frequency-shifted by the carrier offset .DELTA..omega.. Since the
filter unit of the adaptive equalizer realizes a transfer function
1/{G(.omega.-.DELTA..omega.)H(.omega.-.DELTA..omega.)} to equalize
the input signal, it produces an output signal: ##EQU1## The
spectrum of the received symbols is shifted by the carrier offset
.DELTA..omega.. An inverse Fourier transform of the output signal
is expressed by:
where i=0, 1, 2, 3, . . .
T: symbol interval (sec).
Therefore, the received symbols in the output signal from the
filter unit rotate at the angular velocity .DELTA..omega. without
stopping at rest, and hence the equalization error signal contains
the carrier offset .DELTA..omega..
Accordingly, even the adaptive equalizer cannot compensate for the
carrier offset. It is one of the tasks to be achieved in developing
digital mobile communications receivers to provide appropriate
means for compensating for a carrier offset.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an
adaptive equalizer having means for compensating for a carrier
offset between the frequency of a carrier used for
quasi-synchronous detection and the frequency of a transmitted
carrier in a digital mobile communications receiver.
To achieve the above object, the principles of the present
invention are based on the fact that an equalization error signal
of an adaptive equalizer contains a carrier offset, and
equalization error signals are accumulated to extract the carrier
offset to compensate for the carrier offset.
According to a first aspect of the present invention, there is
provided an adaptive equalizer comprising a filter unit for
compensating for a signal distortion to which input digital data
supplied to the filter unit is subjected, correcting means for
correcting output data from the filter unit based on corrective
data, a decision unit responsive to output data from the correcting
means for determining and outputting symbols of the digital data,
first inversely correcting means for inversely correcting output
data from the decision unit which have been corrected by the
correcting means, second inversely correcting means for inversely
correcting difference output data produced by subtracting the
output data of the decision unit from the output data of the
correcting means and corrected by the correcting means, coefficient
updating means for updating filter coefficients of the filter unit
based on output data from the second inversely correcting means,
and frequency offset estimating means for estimating corrective
data based on an offset of a carrier frequency in the output data
from the filter unit on the basis of the output data from the
second inversely correcting means, and outputting the estimated
corrective data as the corrective data to the correcting means.
The correcting means may comprise a complex multiplier for
multiplying the output data from the filter unit by the corrective
data.
The first inversely correcting means may comprise complex conjugate
converting means for converting the corrective data into complex
conjugate data, and a first complex multiplier for multiplying
output data from the complex conjugate converting means by the
output data from the decision unit, and the second inversely
correcting means may comprise a second complex multiplier for
multiplying the output data from the complex conjugate converting
means by the difference output data.
The frequency offset estimating means may comprise converting means
for converting the output data from the second inversely correcting
means into a phase quantity based on the output data from the
second inversely correcting means, a first accumulator/adder for
accumulating and adding phase quantities outputted from the
converting means for a predetermined period, a second
accumulator/adder for accumulating and adding output data from the
first accumulator/adder, and a vector data converter for converting
output data from the second accumulator/adder into unit vector
data, as the corrective data, which has a phase angle based on the
output data from the second accumulator/adder.
According to a second aspect of the present invention, there is
provided an adaptive equalizer comprising a filter for compensating
for a transmission distortion to which an input signal comprising a
training signal and data following the training signal is
subjected, decision means for estimating and outputting symbols of
the input signal in response to an output signal from the filter,
training signal generating means for outputting a training signal,
first selecting means for selecting one of an output signal from
the decision means and the training signal outputted from the
training signal generating means, coefficient updating means for
controlling the coefficients of the filter based on an output
signal which is produced by subtracting an output signal of the
first selector means from the output signal of the filter, memory
means for storing the input signal, a modulator for successively
modulating signals, as a carrier, having a plurality of frequencies
within a predetermined frequency range containing an expected
frequency offset with the training signal read from the memory
means, second selecting means for supplying a modulated output
signal from the modulator to the filter, adding means for adding an
output signal, per carrier frequency, which is produced by
subtracting the output signal of the filter supplied with the
training signal outputted through the second selecting means, from
the training signal outputted through the first selecting means,
and correcting means for converting the carrier frequency
corresponding to a minimum value of a sum output signal from the
adding means into a corrective signal based on the carrier
frequency, and correcting the input signal with the corrective
signal.
The correcting means may comprise converting means for converting
the carrier frequency corresponding to the minimum value of the sum
output signal from the adding means into a unit vector having a
phase angle based on the carrier frequency, and a complex
multiplier for multiplying the input signal read from the memory
means by the unit vector outputted from the converting means, and
supplying a product output signal through the second selecting
means to the filter.
The correcting means may comprise converting means for converting
the carrier frequency corresponding to the minimum value of the sum
output signal from the adding means into a voltage based on the
carrier frequency, and a local oscillator in a stage preceding the
adaptive equalizer for controlling an oscillating frequency in
response to the voltage which is supplied as a corrective control
voltage from the converting means.
According to a third aspect of the present invention, there is
provided an adaptive equalizer comprising memory means for storing
an input signal comprising a training signal and data following the
training signal, a modulator for successively modulating a
plurality of frequencies, as a carrier, within a predetermined
frequency range containing an expected frequency offset with the
training signal read from the memory means, a filter for
compensating for a transmission distortion to which the input
signal is subjected, first selecting means for selecting and
supplying one of an output signal from the modulator and stored
contents of the memory means to the filter, correcting means for
correcting an output signal from the filter with corrective data,
decision means for estimating and outputting symbols of the input
signal in response to an output signal from the correcting means,
training signal generating means for outputting a training signal,
second selecting means for selecting one of an output signal from
the decision means and the training signal outputted from the
training signal generating means, inversely correcting means for
inversely correcting a difference output signal produced by
subtracting an output signal of the second selecting means from the
output signal of the correcting means and corrected by the
correcting means, coefficient updating means for updating the
coefficients of the filter based on an output signal from the
inversely correcting means, initial value estimating means for
adding, per carrier frequency, the output signal from the inversely
correcting means when the output signal from the modulator is
selected by the first selecting means and the training signal
outputted from the training signal generating means is selected by
the second selecting means, and converting a carrier frequency
corresponding to a minimum value of a sum into corrective data as
an estimated initial value based on the carrier frequency, and
frequency offset estimating means for estimating corrective data
based on the output signal from the inversely correcting means,
using the estimated initial value from the initial value estimating
means as an initial corrective value, when the stored contents of
the memory means are selected by the first selecting means and the
output signal from the decision means is selected by the second
selecting means, and outputting the estimated corrective data to
the correcting means.
According to a fourth aspect of the present invention, there is
provided an adaptive equalizer comprising memory means for storing
an input signal comprising a training signal and data following the
training signal, a modulator for successively modulating a
plurality of frequencies, as a carrier, within a predetermined
frequency range containing an expected frequency offset with the
training signal read from the memory means, a filter for
compensating for a transmission distortion to which the input
signal is subjected, first selecting means for selecting and
supplying one of an output signal from the modulator and stored
contents of the memory means to the filter, correcting means for
correcting an output signal from the filter with corrective data,
decision means for estimating and outputting symbols of the input
signal in response to an output signal from the correcting means,
training signal generating means for outputting a training signal,
second selecting means for selecting one of an output signal from
the decision means and the training signal outputted from the
training signal generating means, first inversely correcting means
for inversely correcting an output signal from the second selecting
means which is corrected by the correcting means, and feeding back
the inversely corrected output signal to the filter, second
inversely correcting means for inversely correcting a difference
output signal produced by subtracting an output signal of the
second selecting means from the output signal of the correcting
means and corrected by the correcting means, coefficient updating
means for updating the coefficients of the filter based on an
output signal from the second inversely correcting means, initial
value estimating means for adding, per carrier frequency, the
output signal from the second inversely correcting means when the
output signal from the modulator is selected by the first selecting
means and the training signal outputted from the training signal
generating means is selected by the second selecting means, and
converting a carrier frequency corresponding to a minimum value of
a sum into corrective data as an estimated initial value based on
the carrier frequency, and frequency offset estimating means for
estimating corrective data based on the output signal from the
second inversely correcting means, using the estimated initial
value from the initial value estimating means as an initial
corrective value, when the stored contents of the memory means are
selected by the first selecting means and the output signal from
the decision means is selected by the second selecting means, and
outputting the estimated corrective data to the correcting
means.
With the adaptive equalizer according to the first aspect of the
present invention, the signal distortion is compensated for by the
filter unit, the output data from the filter unit are corrected
based on the corrective data estimated by the frequency offset
estimating means, and symbols of the output data from the filter
unit which are corrected are estimated and outputted as output data
from the decision unit. The output data from the decision unit
which have been corrected by the correcting means are inversely
corrected by the first inversely correcting means into uncorrected
output data from the filter unit, which are fed back to a feedback
filter of the filter unit. Equalization error data are calculated
by subtracting the output data of the decision unit from the output
data of the filter unit, and then inversely corrected by the second
inversely correcting means into uncorrected equalization error
data. Based on the uncorrected equalization error data, the
coefficients of the filter unit are updated by the coefficient
updating means. Corrective data based on a carrier frequency offset
in the output data from the filter unit are estimated by the
frequency offset estimating means on the basis of the uncorrected
equalization error data, and the output data from the filter unit
are corrected by the estimated corrective data. Therefore, effects
based on transmission characteristics of a transmission path are
corrected, and an error based on the frequency offset is
corrected.
If the frequency offset estimating means comprises the converting
means for converting the output data from the second inversely
correcting means into a phase quantity based on the output data
from the second inversely correcting means, the first
accumulator/adder for accumulating and adding phase quantities
outputted from the converting means for a predetermined period, the
second accumulator/adder for accumulating and adding output data
from the first accumulator/adder, and the vector data converter for
converting output data from the second accumulator/adder into unit
vector data, as the corrective data, which has a phase angle based
on the output data from the second accumulator/adder, then the
output data from the second inversely correcting means, i.e., the
uncorrected equalization error data, are converted into the phase
quantity based on the equalization error data by the converting
means, and the phase quantities are added so that variations based
on the distortion which the input signal has suffered in the
transmission path are averaged and eliminated by being accumulated
and added. The phase quantity based on the frequency offset is
outputted, and the phase quantities are accumulated and added so as
to be converted into the unit vector data having the phase angle
corresponding to the accumulated and added output data. The output
data from the filter unit are corrected based on the unit vector
data, and hence the error based on the frequency offset is
corrected.
With the adaptive equalizer according to the second aspect of the
present invention, the training signal of the input signal is read
from the memory means, and the signals, as a carrier, having a
plurality of frequencies in the predetermined frequency range
containing the expected frequency offset are modulated with the
read training signal by the modulator at carrier frequencies at
given frequency intervals. The modulated output signal is supplied
through the second selecting means to the filter, which compensates
for the transmission distortion. The training signal outputted
through the first selecting means is subtracted from the output
signal from the filter. The difference output signal is added per
carrier frequency, and the minimum value of the sum output signal
is searched for and converted into the corrective signal based on
the carrier frequency corresponding to the minimum value. The input
signal read from the memory means is corrected on the basis of the
converted corrective signal.
If the input signal has suffered an offset of frequency f in the
preceding stage, then the input signal has been rotated by the
phase angle .theta. corresponding to the frequency f. When the
input signal is modulated with a carrier frequency (-f) which
inversely gives a rotation by the phase angle (-.theta.), the
output signal from the filter becomes equal to the training signal
outputted from the training signal generating means, thus canceling
the offset from the modulated output signal. Therefore, the carrier
frequency corresponding to the minimum value of the sum output
signal is the frequency offset. As a result, the frequency offset
is corrected by the above correction.
If the correcting means comprises the converting means for
converting the carrier frequency corresponding to the minimum value
of the sum output signal from the adding means into a unit vector
having a phase angle based on the carrier frequency, and the
complex multiplier for multiplying the input signal read from the
memory means by the unit vector outputted from the converting
means, and supplying a product output signal through the second
selecting means to the filter, then the unit vector outputted from
the converting means has a phase angle (-.theta.), and the
frequency offset is corrected by multiplying, with the complex
multiplier, the unit vector having the phase angle (-.theta.) by
the input signal read from the memory means and rotated through the
phase angle (.theta.) by the frequency offset.
If the correcting means comprises the converting means for
converting the carrier frequency corresponding to the minimum value
of the sum output signal from the adding means into a voltage based
on the carrier frequency, and a quadrature-detection carrier
oscillator as a local oscillator in a stage preceding the adaptive
equalizer for controlling an oscillating frequency in response to
the voltage which is supplied as a corrective control voltage from
the converting means, then the converting means outputs a voltage
corresponding to the frequency -f, and the oscillating frequency of
the local oscillator is corrected by the voltage. Therefore, the
frequency offset of the frequency f which has occurred in the stage
preceding the adaptive equalizer can be corrected.
With the adaptive equalizer according to the third aspect of the
present invention, the output signal from the modulator is selected
by the first selecting means, and the training signal outputted
from the training signal generating means is selected by the second
selecting means. The training signal of the input signal is read
from the memory means, and a plurality of frequencies, as a
carrier, in a predetermined frequency range containing an expected
frequency offset are successively modulated by the training signal
read from the memory means at predetermined frequency intervals.
The modulated output signal is supplied through the first selecting
means to the filter, which compensates for the transmission
distortion. The training signal outputted through the second
selecting means is subtracted from the output signal from the
filter. The difference output signal is added per carrier
frequency, and converted into the corrective data, as an estimated
initial value, based on the carrier frequency corresponding to the
minimum value.
Then, the stored contents read from the memory means are selected
by the first selecting means, and the output signal from the
decision means is selected by the second selecting means. The
stored contents read from the memory means and outputted through
the first selecting means are supplied to the filter to compensate
for the transmission distortion. The output signal from the filter
is corrected by the correcting means based on the corrective data
outputted from the frequency offset estimating means. The output
signal outputted from the decision means through the second
selecting means is subtracted from the corrected output signal from
the filter. The difference output signal which has been corrected
by the correcting means is inversely corrected by the inversely
correcting means, and the coefficients of the filter are updated
based on the inversely corrected difference output signal. The
corrective data are estimated on the basis of the inversely
corrected difference output signal. Specifically, the corrective
data are estimated using the estimated initial value as an initial
value, and the estimated corrective data are outputted as
corrective data to the correcting means for use in correcting the
output signal from the filter. Since the estimated initial value is
used as an initial value for estimating the corrective data, the
adaptive equalizer can compensate for a frequency offset in a wide
range.
With the adaptive equalizer according to the fourth aspect of the
present invention, the output signal from the modulator is selected
by the first selecting means, and the training signal outputted
from the training signal generating means is selected by the second
selecting means. The training signal of the input signal is read
from the memory means, and a plurality of frequencies, as a
carrier, in a predetermined frequency range containing an expected
frequency offset are successively modulated by the training signal
read from the memory means at predetermined frequency intervals.
The modulated output signal is supplied through the first selecting
means to the filter, which compensates for the transmission
distortion. The training signal outputted through the second
selecting means is subtracted from the output signal from the
filter. The difference output signal is added per carrier
frequency, and converted into the corrective data, as an estimated
initial value, based on the carrier frequency corresponding to the
minimum value.
Then, the stored contents read from the memory means are selected
by the first selecting means, and the output signal from the
decision means is selected by the second selecting means. The
stored contents read from the memory means and outputted through
the first selecting means are supplied to the filter to compensate
for the transmission distortion. The output signal from the filter
is corrected by the correcting means based on the corrective data
outputted from the frequency offset estimating means. The output
signal outputted from the decision means through the second
selecting means which has been corrected by the correcting means is
inversely corrected by the first inversely correcting means, and
the inversely corrected output signal from the decision means is
fed back to the filter. As a result, a multipath distortion is
removed.
The output signal outputted from the decision means through the
second selecting means is subtracted from the corrected output
signal from the filter. The difference output signal which has been
corrected by the correcting means is inversely corrected by the
second inversely correcting means,.and the coefficients of the
filter are updated based on the inversely corrected difference
output signal. The corrective data are estimated on the basis of
the inversely corrected difference output signal. Specifically, the
corrective data are estimated using the estimated initial value as
an initial value, and the estimated corrective data are outputted
as corrective data to the correcting means for use in correcting
the output signal from the filter. Since the estimated initial
value is used as an initial value for estimating the corrective
data, the adaptive equalizer can compensate for a frequency offset
in a wide range.
In the adaptive equalizer according to the third or fourth aspect
of the present invention, the frequency offset estimating means may
comprise converting means for converting the output signal from the
inversely correcting means (second inversely correcting means) into
a phase quantity based on the output signal from the inversely
correcting means (second inversely correcting means), first
accumulating/adding means for accumulating and adding phase
quantities outputted from the converting means for a predetermined
period, second accumulating/adding means for accumulating and
adding output phase quantities from the first accumulating/adding
means for a predetermined period, and a vector data converter for
converting an output phase quantity from the second
accumulating/adding means into a unit vector, as the corrective
data, which has a phase angle based on the output phase quantity
from the second accumulating/adding means. In such an arrangement,
the output data from the inversely correcting means (second
inversely correcting means), i.e., the uncorrected difference
output signal, are converted into the phase quantity based on the
difference output signal, and the phase quantities are added so
that variations based on the distortion which the input signal has
suffered in the transmission path are averaged and eliminated by
being accumulated and added. The phase quantity based on the
frequency offset is outputted, and the phase quantities are
accumulated and added by the second accumulating/adding means so as
to be converted into the unit vector data having the phase angle
corresponding to the accumulated and added output data from the
second accumulating/adding means. Since the output signal from the
filter is corrected based on the unit vector data, the error based
on the frequency offset is corrected.
The above and other objects, features, and advantages of the
present invention will become apparent from the following
description when taken in conjunction with the accompanying
drawings which illustrate preferred embodiments of the present
invention by way of example.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an adaptive equalizer according to a
first embodiment of the present invention;
FIG. 2 is a block diagram of a frequency offset estimating unit in
the adaptive equalizer according to the first embodiment;
FIG. 3 is a diagram showing the format of input signal data;
FIG. 4 is a block diagram of an adaptive equalizer according to a
second embodiment of the present invention;
FIG. 5 is a block diagram of an adaptive equalizer according to a
third embodiment of the present invention;
FIG. 6 is a block diagram of an adaptive equalizer according to a
fourth embodiment of the present invention;
FIG. 7 is a block diagram of a frequency offset estimating unit in
the adaptive equalizer according to the fourth embodiment;
FIG. 8 is a block diagram of a conventional digital cellular
receiver; and
FIG. 9 is a block diagram of an adaptive equalizer in the
conventional digital cellular receiver.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
1st Embodiment
An adaptive equalizer according to a first embodiment of the
present invention, as shown in FIG. 1, may replace the adaptive
equalizer in the conventional digital cellular receiver shown in
FIG. 8. Therefore, the I and Q components of an output signal of
the quadrature detector shown in FIG. 8 are supplied through the
A/D converter and the demultiplexer to the adaptive equalizer shown
in FIG. 1. In the adaptive equalizer, the I and Q components are
supplied as a complex signal whose real part composed of the I
component and imaginary part composed of the Q component to a
filter unit 11.
The filter unit 11 comprises a feed-forward filter 110 comprising
unit delay operators 111, 112, complex coefficient multipliers 113,
114, 115, and a complex adder 116, and a feedback filter 120
comprising a unit delay operator 117 and a complex coefficient
multiplier 118.
An output signal from the complex adder 116, i.e., a filter output
signal (a.sub.of, b.sub.of) from the feed-forward filter 110, is
supplied to a complex multiplier 12 which multiplies the filter
output signal by a reversely rotating unit vector representing an
estimated carrier offset that is estimated by a carrier offset
estimating unit 20A for thereby canceling out a carrier offset
contained in the output signal from the filter unit 11.
The complex multiplier 12 supplies its output signal to a decision
unit 13 that determines which phase angle range of the equation (2)
the distorted phase angle of the output signal from the filter unit
11 falls in. The decision unit 13 then outputs a signal having the
I and Q components with a standardized phase angle expressed by the
equation (2).
The output signal from the decision unit 13 is also supplied to a
terminal 152 of a selector switch 15, and subtracted from the
output signal from the complex multiplier 12 by a complex
subtractor 17, thus producing an equalization error signal. The
equalization error signal is a vector having, as its phase angle,
the difference between the above distorted phase angle and the
standardized phase angle expressed by the equation (2).
A training signal generated by a training signal generator 14 is
supplied to the other terminal 151 of the selector switch 15. The
selector switch 15 can thus select the training signal from the
training signal generator 14 or the output signal from the decision
unit 13.
The reversely rotating unit vector representing the estimated
carrier offset estimated by the carrier offset estimating unit 20A
is supplied to a complex conjugate converter 21 as well as the
complex multiplier 12. The complex conjugate converter 21 converts
the supplied unit vector into its conjugate complex signal, which
is supplied to a complex multiplier 16 that multiplies the supplied
conjugate complex signal by the output signal from the decision
unit 13, thereby reversely correcting the corrected signal which
has been corrected based on the estimated carrier offset by the
complex multiplier 12, i.e., introducing the carrier offset again.
The complex multiplier 16 supplies its reversely corrected output
signal to the unit delay operator 117 of the feedback filter
120.
The equalization error signal from the complex subtractor 17 is
also supplied to a complex multiplier 18, which multiplies it by
the output signal from the complex conjugate converter 21, thereby
inversely correcting the equalization error signal which has been
corrected based on the estimated carrier offset by the complex
multiplier 12, i.e., introducing the carrier offset again. The
complex multiplier 18 supplies its inversely corrected output
signal to a coefficient updating unit 19. The coefficient updating
unit 19 updates the coefficients of the coefficient multipliers
113.about.115, 118 of the filter unit 11.
The output signal from the complex multiplier 18 is also applied to
a controller 22A and also the carrier offset estimating unit 20A.
The controller 22A accumulates the levels of equalization error
signals for a certain period of time to monitor the operation of
the adaptive equalizer. If the accumulated error is smaller than a
predetermined value, then the controller 22A determines that the
adaptive equalizer is operating normally, and turns on a switch
200A (see FIG. 2). If the accumulated error is in excess of the
predetermined value, then the controller 22A determines that the
adaptive equalizer is malfunctioning, and turns off the switch 200A
thereby stopping the estimating operation of the carrier offset
estimating unit 20A.
As shown in FIG. 2, the carrier offset estimating unit 20A
comprises the switch 200A, an equalization error-to-phase converter
201A for converting the inversely corrected equalization error
signal supplied from the complex multiplier 18 through the switch
200A into a phase quantity, an accumulator/adder 202A for
accumulating and adding phase quantities converted by the
equalization error-to-phase converter 201A, an accumulator/adder
203A for accumulating and adding output signals from the
accumulator/adder 202A, and a phase-to-vector converter 204A for
converting an output signal from the accumulator/adder 203A into a
reversely rotating unit vector having a corresponding phase angle
and supplying the reversely rotating unit vector to the complex
multiplier 12.
The carrier offset estimating unit 20A operates as follows: The
inversely corrected equalization error signal supplied from the
complex multiplier 18 through the switch 200A, i.e., a signal
containing noise in the transmission path, phase and amplitude
variations due to fading, and a phase rotation due to the carrier
offset, is converted into a phase quantity by the equalization
error-to-phase converter 201A.
Then, signals indicative of phase quantities from the equalization
error-to-phase converter 201A are accumulated and added by the
accumulator/adder 202A. When the signals indicative of phase
quantities are accumulated and added for a certain period of time,
the accumulated values of noise in the transmission path and phase
and amplitude variations due to fading become zero, and the phase
rotation due to the carrier offset amounts to an angular velocity
.phi..sub.s.
Output signals from the accumulator/adder 202A are also accumulated
and added by the accumulator/adder 203A for the time of a
one-symbol interval, and converted into a phase rotation quantity
k.multidot..phi..sub.s per symbol (k represents the time of the
one-symbol interval).
The phase rotation quantity k.multidot..phi..sub.s is then
converted by the phase-to-vector converter 204A into a unit vector
which has the phase rotation quantity and is reversely rotating.
Thereafter, the reversely rotating unit vector is supplied to the
complex multiplier 12, which reversely rotates the phase of the
output signal from the filter unit 11 to cancel out the carrier
offset.
If the input signal contains an excessive carrier offset and cannot
be corrected by the output signal from the carrier offset
estimating unit 20A, and the equalization error signal supplied to
the controller 22A exceeds a predetermined level, then the
controller 22A applies a control signal to turn off the switch
200A. At this time, the carrier offset estimating unit 20A outputs
a unit vector having a phase angle that has been estimated in the
preceding slot.
As described above, the adaptive equalizer according to the first
embodiment of the present invention compensates for a distorted
component, i.e., a fading component, produced in the transmission
path, and also estimates a carrier offset from the equalization
error signal and corrects the output signal from the filter unit
based on the estimated carrier offset. The adaptive equalizer can
thus compensate for a phase rotation caused by the carrier offset
which results from the difference between the frequencies of the
detecting carrier and the transmitted carrier in the quadrature
detector, for thereby preventing the bit error rate from being
deteriorated by the carrier offset.
In estimating the carrier offset, the distorted component that
varies with time in the transmission path is removed by
accumulation and addition for a certain period of time for
extracting a phase quantity based on the carrier offset.
Consequently, the carrier offset estimating unit is relatively
simple in arrangement.
2nd Embodiment
FIG. 4 shows an adaptive equalizer according to a second embodiment
of the present invention. Those parts shown in FIG. 4 which are
identical to those shown in FIG. 1 are denoted by identical
reference numerals and will not be described in detail below.
As shown in FIG. 3, an input signal applied to the adaptive
equalizer according to the second embodiment comprises a training
signal 2 and data 3. As shown in FIG. 4, the training signal 2 and
the data 3 are stored in a buffer memory 23. Then, the training
signal stored in the buffer memory 23 is read and supplied to a
digital modulator 24, which digitally modulates the training signal
with carrier frequencies at the interval of a predetermined
frequency .DELTA.f within a carrier offset range of from -f.sub.off
to +f.sub.off which is intentionally added. The digital modulator
24 outputs and applies the digital modulated signal to a terminal
251 of a selector switch 25, which can select the modulated
signal.
After the training signal is read a predetermined number of times
from the buffer memory 23, the training signal and the data stored
in the buffer memory 23 are read and supplied to the complex
multiplier 12, which multiplies them by estimated corrective data
(described later on). The complex multiplier 12 supplies its output
signal to the other terminal 252 of the selector switch 25, which
can select the output signal from the complex multiplier 12. The
digital modulated signal from the digital modulator 24 or the
output signal from the complex multiplier 12, which is selected by
the selector switch 25, is supplied to the filter unit 11.
The filter unit 11 is identical to the filter unit 11 according to
the first embodiment. The output signal from the filter unit 11 is
applied to the decision unit 13 that determines the phase of the
output signal from the filter unit 11. The decision unit 13
supplies its output signal to the terminal 152 of the selector
switch 15.
The training signal generated by the training signal generator 14
is supplied to the terminal 151 of the selector switch 15. The
selector switch 15 can thus select the training signal from the
training signal generator 14 or the output signal from the decision
unit 13.
The selected output data from the selector switch 15 are fed back
to the unit delay operator 117, i.e., the feedback filter 119 of
the filter unit 11. The selected output data from the selector
switch 15 is also subtracted from the output data from the filter
unit 11, thus producing equalization error data. The equalization
error data are supplied to the coefficient updating unit 19, which
updates the coefficients of the coefficient multipliers
113.about.115, 118 of the filter unit 11, i.e., updates the filter
coefficients based on the equalization error data.
The equalization error data are also supplied through a terminal
271 of a selector switch 27 to an equalization error
calculator/memory 26. The equalization error calculator/memory 26
has a calculating unit for adding equalization error data with
respect to all symbols supplied to the digital modulator 24 for
each of the carrier frequencies, and a memory unit for storing the
sum for each of the carrier frequencies.
The sum of equalization error data stored in the equalization error
calculator/memory 26 is read into a carrier offset estimating unit
20B, which detects the minimum value of the sum, converts the
detected minimum value into a phase angle, and adds the converted
phase angle each time a symbol is read, for conversion into a unit
vector with the converted phase angle. The carrier offset
estimating unit 20B supplies the unit vector as corrective data to
the complex multiplier 12 in synchronism with the reading of the
symbols.
The selector switches 25, 15, 27 are controlled by a control
circuit 28 to shift in different patterns in different modes
(described below).
Operation of the adaptive equalizer according to the second
embodiment will be described below with reference to Tables 1
through 4 below.
When the input signal 1 having a format as shown in FIG. 3 is
supplied to the buffer memory 23, it is stored in the buffer memory
23. An initializing mode and a carrier offset estimating mode for
the filter unit 11 are carried out using the training signal 2, and
an adaptive equalizing mode is carried out using the training
signal 2 and the data 3 according to the sequences described below.
As described above, the range of carrier frequencies intentionally
applied to the digital modulator 24 is the same as the range
(-f.sub.off to +f.sub.off (Hz)) of the carrier offset which the
received series of symbols is supposed to suffer, and the carrier
frequencies change at the interval of the frequency .DELTA.f
(Hz).
(a) First, the initializing mode is carried out.
The initializing mode and the carrier offset estimating mode are
related to each other as follows:
The filter coefficients are initialized with respect to all the
symbols of the training signal for one carrier frequency, and then
the carrier offset estimating mode is carried out with respect to
all the symbols of the training signal for the same carrier
frequency. Thereafter, the carrier frequency is varied by the
frequency .DELTA.f, and then the filter coefficients are
initialized and the carrier offset estimating mode is carried
out.
Table 1 below shows the manner in which the initializing mode is
carried out.
TABLE 1
__________________________________________________________________________
Read sig- Initializing mode nal Training signal
__________________________________________________________________________
Carrier -f.sub.off -f.sub.off + -f.sub.off + . . . +f.sub.off
frequency .DELTA.f 2.DELTA.f Operating Setting Updating, Updating,
Updating, Updating, phase I of FC Setting Setting Setting Setting
of FC of FC of FC of FC Operating -- -- -- -- -- phase II Operating
-- -- -- -- -- phase III Connected 25: 251 25: 251 25: 251 25: 251
25: 251 terminals 15: 151 15: 151 15: 151 15: 151 15: 151 of SW 25,
27: Open 27: Open 27: Open 27: Open 27: Open 15, 27
__________________________________________________________________________
FC: Filter coefficients
(a-0) During the initializing mode, the selector switch 25 is
connected to the terminal 251, the selector switch 15 is connected
to the terminal 151, and the selector switch 27 is open under the
control of the control circuit 28.
(a-1) The training signal of the input signal 1 stored in the
buffer memory 23 is read and applied to the digital modulator 24,
which modulates the training signal with a carrier frequency f. The
modulated signal is supplied to the filter unit 11, which removes
an intersymbol interference from the modulated signal by
filtering.
(a-2) The output data from the filter unit 11 is supplied to the
complex subtractor 17, which calculates the difference between the
output data from the filter unit 11 and the training signal that is
generated by the training signal generator 14 depending on the
training signal read from the buffer memory 23, i.e., equalization
error data.
(a-3) The equalization error data calculated by the complex
subtractor 17 are supplied to the coefficient updating unit 19
thereby to update the filter coefficients of the filter unit 17
based on the equalization error data.
(a-4) The training operation based on the carrier frequency f is
effected with respect to all the symbols of the training signal.
When the training operation is finished, the filter coefficients
are converged.
The initializing mode based on the carrier frequency f is followed
by the carrier offset estimating mode that is carried out at the
carrier frequency f. Table 2 below shows the manner in which the
carrier offset estimating mode is carried out.
TABLE 2
__________________________________________________________________________
Read sig- Carrier offset estimating mode nal Training signal
__________________________________________________________________________
Carrier -f.sub.off -f.sub.off + -f.sub.off + . . . +f.sub.off
frequency .DELTA.f 2.DELTA.f Operating Setting Updating, Updating,
Updating, Updating, phase I of FC Setting Setting Setting Setting
of FC of FC of FC of FC Operating Accumu- Accumu- Accumu- Accumu-
Accumu- phase II lating, lating, lating, lating, lating, adding of
adding of adding of adding of adding of equal- equal- equal- equal-
equal- ization ization ization ization ization error error error
error error data data data data data Operating Searching for
minimum value .fwdarw. phase III Estimating phase angle Connected
25: 251 25: 251 25: 251 25: 251 25: 251 terminals 15: 151 15: 151
15: 151 15: 151 15: 151 of SW 25, 27: 271 27: 271 27: 271 27: 271
27: 271 15, 27
__________________________________________________________________________
FC: Filter coefficients
(b-0) During the carrier offset estimating mode, the selector
switch 25 is connected to the terminal 251, the selector switch 15
is connected to the terminal 151, and the selector switch 27 is
connected to the terminal 271 under the control of the control
circuit 28. In the carrier offset estimating mode, the filter
coefficients obtained in the initializing mode are used as initial
values.
(b-1) The training signal of the input signal 1 stored in the
buffer memory 23 is read and applied to the digital modulator 24,
which modulates the training signal with a carrier frequency f. The
modulated signal is supplied to the filter unit 11.
(b-2) The output data from the filter unit 11 is supplied to the
complex subtractor 17, which calculates the difference between the
output data from the filter unit 11 and the training signal that is
generated by the training signal generator 14 depending on the
training signal read from the buffer memory 23, i.e., equalization
error data.
(b-3) The equalization error data calculated by the complex
subtractor 17 are supplied to the coefficient updating unit 19
thereby to update the filter coefficients of the filter unit 17
based on the equalization error data.
(b-4) At the same time, the equalization error data are supplied to
the equalization error calculator/memory 26, which calculates, in
its calculating unit, the sum of equalization error data with
respect to all the symbols at the carrier frequency f supplied to
the digital modulator 24. The calculated sum is stored in the
memory unit of the equalization error calculator/memory 26 with
respect to the carrier frequency f supplied to the digital
modulator 24.
(c-0) Then, the carrier frequency f is increased by the frequency
.DELTA.f. The initializing mode from (a-0) to (a-4) is carried out
at the frequency (f+.DELTA.f), and then the carrier offset
estimating mode from (b-0) to (b-4) is carried out at the frequency
(f+.DELTA.f).
(c-1) The above process is repeated as the carrier frequency is
varied from -f.sub.off to +f.sub.off at the interval .DELTA.f.
The stored contents of the equalization error calculator/memory 26
upon completion of the above operation are given in Table 3
below.
TABLE 3 ______________________________________ Frequency f
Accumulated and added equalization errors
______________________________________ -f.sub.off .alpha..sub.1
-f.sub.off + .DELTA.f .alpha..sub.2 -f.sub.off + 2.DELTA.f
.alpha..sub.3 . . . . . . +f.sub.off - .DELTA.f .alpha..sub.n+1
+f.sub.off .alpha..sub.n ______________________________________
(c-2) When the operation up to the carrier frequency +f.sub.off is
over, the carrier offset estimating unit 22 searches for the
minimum value of all accumulated and added equalization error data
stored in the equalization error calculator/memory 26, converts the
minimum value into a phase angle based on a carrier frequency
corresponding to the minimum value, and converts the converted
phase angle into a unit vector having the converted phase angle.
The unit vector is sent as corrective data to the complex
multiplier 12. The unit vector having the phase angle (-.theta.)
based on the minimum value of all accumulated and added
equalization error data is sent as corrective data to the complex
multiplier 12 for the reasons as follows:
If the input signal 1 has suffered an offset of frequency f in the
preceding stage, then the input signal has been rotated by the
phase angle .theta.. When the input signal is modulated with a
carrier frequency -f which inversely gives a rotation by the phase
angle (-.theta.), the output signal from the filter unit 11 becomes
equal to the training signal outputted from the training signal
generator 14, thus canceling the offset from the modulated output
signal.
Therefore, the input signal which has been rotated by the phase
angle .theta. can be corrected by being multiplied by the unit
vector having a phase angle (-.theta..sub.1) based on a frequency
-f.sub.1 (.apprxeq.-f) where the sum of equalization error data is
minimum.
Then, the adaptive equalizing mode is carried out following the
completion of the carrier offset estimating mode. Table 4 below
shows the manner in which the adaptive equalizing mode is carried
out.
TABLE 4 ______________________________________ Read sig- Adaptive
equalizing mode nal Training signal Data
______________________________________ Carrier -- -- frequency
Operating Updating, Setting of FC Updating, Setting of FC phase I
Operating Correction of each Correction of data with phase II
training signal with estimated phase angle initial phase angle
Operating -- -- phase III Connected 25: 252 25: 252 terminals 15:
151 15: 152 of SW 25, 27: Open 27: 271 15, 27
______________________________________ FC: Filter coefficients
(d-0) During the adaptive equalizing mode, the selector switch 15
is connected to the terminal 151 in a training signal period under
the control of the control circuit 28, and the selector switch 15
is connected to the terminal 152, the selector switch 25 is
connected to the terminal 252, and the selector switch 27 is open
in a data equalizing period under the control of the control
circuit 28. In the adaptive equalizing mode, the filter
coefficients obtained in the carrier offset estimating mode are
used as initial values.
(d-1) A training signal of the input signal 1 stored in the buffer
memory 23 is read and applied to the complex multiplier 12, which
multiplies the training signal by the initial unit vector as
corrective data. The corrected signal is then filtered by the
filter unit 11.
(d-2) The output data from the filter unit 11 is supplied to the
complex subtractor 17, which calculates the difference between the
output data from the filter unit 11 and the training signal that is
generated by the training signal generator 14 depending on the
training signal read from the buffer memory 23, i.e., equalization
error data.
(d-3) The equalization error data calculated by the complex
subtractor 17 are supplied to the coefficient updating unit 19
thereby to update the filter coefficients of the filter unit 17
based on the equalization error data.
(d-4) Following training signals are successively read, and the
above process (d-1).about.(d-3) is effected until all the training
signals are finished. Each time the next training signal is read,
the phase angle of the unit vector is rotated by the phase angle of
the unit vector as the initial value in the carrier offset
estimating unit 20B. The unit vector whose phase angle is
multiplied by an integer is supplied as corrective data to the
complex multiplier 12, which multiplies the output data from the
filter unit 11 by the corrective data, thereby correcting the
output data from the filter unit 11.
In the carrier offset estimating unit 20B, the converted phase
angle is added each time a symbol is read because the phase angle
of the input signal which has suffered the carrier offset is
incremented by the phase angle .theta. per input signal so that it
varies from .theta. to 2.theta. to 3.theta. . . . . In the adaptive
equalizing mode, the phase angle (-.theta..sub.1) of the corrective
data, i.e., the unit vector, is incremented by (-.theta..sub.1)
each time the data is read from the buffer memory 23 so that the
phase angle varies from (-.theta..sub.1) to (-2.theta..sub.1) to
(-3.theta..sub.1) to (-4.theta..sub.1) . . . .
(d-5) After the reading of all training signals is finished, the
selector switch 25 is connected to the terminal 152, and the data
are successively read from the buffer memory 23. The read data are
corrected by being multiplied by the unit vector which is rotated
by the phase angle .theta..sub.1 each time the data is read and
which is outputted from the carrier offset estimating unit 20B, and
the corrected data are supplied to and filtered by the filter unit
11.
(d-6) The output data from the filter unit 11 are supplied to the
complex subtractor 17, which calculates the difference between the
output data from the filter unit 11 and the output data from the
decision unit 13 to produce equalization error data.
(d-7) The equalization error data calculated by the complex
subtractor 17 are supplied to the coefficient updating unit 19
thereby to update the filter coefficients of the filter unit 17
based on the equalization error data for an adaptive equalizing
process.
(d-8) The above operation is repeated until all the data are read
from the buffer memory 23. Consequently, the carrier offset
produced in the preceding stage such as a radio transmission unit
can be compensated for.
In the adaptive equalizer according to the second embodiment, as
described above, signals, as a carrier, having a plurality of
frequencies in a predetermined frequency range including an
expected carrier offset are successively modulated by the training
signals of the input signal that are read from the buffer memory,
and the modulated output signals are supplied to the filter unit.
Output signals produced by subtracting the training signal
outputted by the training signal generator from the output signals
from the filter unit are added for each carrier frequency. The
carrier frequency corresponding to the minimum value of the sum
output signal is converted into a signal based on the carrier
frequency, and the input signal is equalized by the converted
signal. Therefore, the carrier offset which the input signal has
suffered can be compensated for to prevent the error rate from
being deteriorated.
3rd Embodiment
FIG. 5 shows an adaptive equalizer according to a third embodiment
of the present invention.
The adaptive equalizer according to the third embodiment differs
from the adaptive equalizer according to the second embodiment in
that the complex multiplier 12 of the second embodiment is
dispensed with, and the carrier offset estimating unit 20B of the
second embodiment is replaced with a carrier offset estimating unit
20C and a voltage-controlled oscillator 29 which controls an
oscillating frequency with the sum of a voltage outputted from the
carrier offset estimating unit 20C and a voltage applied from
another source (not shown), the voltage-controlled oscillator (VCO)
29 serving as a local oscillator in a frequency conversion stage or
a quadrature-detection carrier oscillator. The other details of the
adaptive equalizer according to the third embodiment are identical
to those of the adaptive equalizer according to the second
embodiment.
The carrier offset estimating unit 20C is of the same arrangement
as the carrier offset estimating unit 20B. The carrier offset
estimating unit 20C searches for a minimum frequency and converts
the minimum frequency into a corresponding voltage. The voltage
thus produced is maintained at a constant level irrespective of the
reading of the training signal and the data from the buffer memory
23. The voltage is added to the voltage applied from the other
source, and the sum voltage is applied as a control voltage to the
voltage-controlled oscillator 29.
The adaptive equalizer according to the third embodiment operates
in the same manner as the adaptive equalizer according to the
second embodiment with respect to the initializing mode and the
carrier offset estimating mode. In the adaptive equalizing mode,
however, the converted voltage corresponding to the minimum
frequency is maintained at a constant level irrespective of the
reading of the training signal and the data from the buffer memory
23, and added to the voltage applied from the other source, and the
sum voltage is applied as a control voltage to the
voltage-controlled oscillator 29 for controlling the oscillating
frequency.
Therefore, the oscillating frequency of the local oscillator in the
frequency conversion stage is corrected based on the voltage
outputted from the carrier offset estimating unit 20C to compensate
for the carrier offset.
Furthermore, the adaptive equalizer according to the third
embodiment converts the carrier frequency corresponding to the
minimum value of the sum output signal into the unit vector having
the phase angle based on the carrier frequency, multiplies the
input signal read from the buffer memory by the converted unit
vector, and supplies the product output signal to the filter unit.
The multiplication process compensates for the carrier offset which
the input signal suffers to prevent the error rate from being
deteriorated.
In addition, the adaptive equalizer converts the carrier frequency
corresponding to the minimum value of the sum output signal into
the voltage based on the carrier frequency, and applies the
converted voltage to the local oscillator in the stage preceding
the adaptive equalizer. Consequently, the carrier offset which the
input signal is subjected to is compensated for in the stage
preceding the adaptive equalizer to prevent the error rate from
being deteriorated.
4th Embodiment
FIG. 6 shows an adaptive equalizer according to a fourth embodiment
of the present invention.
According to the fourth embodiment, the buffer memory 23 and the
digital modulator 24 in the second embodiment are incorporated in
the adaptive equalizer according to the first embodiment. The
adaptive equalizer according to the fourth embodiment includes a
carrier offset estimating unit 20D and a controller 22B which are
similar to the carrier offset estimating unit 20A and the
controller 22A, respectively, in the first embodiment, and an
equalization error calculator/memory 26 which is similar to the
equalization error calculator/memory 26 in the second embodiment.
The other components and their operation according to the fourth
embodiment are identical to those according to the first through
third embodiments, and will not be described in detail below.
The adaptive equalizer according to the fourth embodiment
additionally has an initial value estimating unit 30. The initial
value estimating unit 30 is supplied with the sum of equalization
error data added per modulation frequency in the digital modulator
24 and stored in the equalization error calculator/memory 26,
detects the minimum value of the sum, converts the detected minimum
value into a phase angle based thereon, and outputs it as an
initial phase angle.
As with the second embodiment, the adaptive equalizer according to
the fourth embodiment operates in three modes, i.e., the
initializing mode, the carrier offset estimating mode, and the
adaptive equalizing mode. These modes are also identical to those
according to the second embodiment except for details which will be
described below.
During the initializing mode and the carrier offset estimating mode
with the training signals, the carrier offset estimating unit 20D
outputs a unit vector (1+j0), and the complex conjugate converter
21 generates a unit vector (1-j0). Therefore, during the
initializing mode and the carrier offset estimating mode, the
output signal from the filter unit 11 and the equalization error
signal outputted from the complex subtractor 17 are not corrected
or inversely corrected with respect to the carrier offset, and the
coefficients of the filter unit 11 are set and updated (see Table
1).
The equalization error calculator/memory 26 does not operate in the
initializing mode, but operates in a certain manner in the carrier
offset estimating mode. More specifically, in the range of
modulation frequencies from -f.sub.off to +f.sub.off in the digital
modulator 24, the sum of equalization error data is accumulated in
the equalization error calculator/memory 26 per frequency interval
.DELTA.f. After the sum has been accumulated in the entire
modulation frequency range, the initial value setting unit 30
detects the minimum value of the sum of equalization error data
stored in the equalized error calculator/memory 26 (see Tables 2
and 3). The initial value setting unit 30 also generates an initial
phase angle (-.theta.) corresponding to the detected minimum value
of the sum of equalization error data.
Then, the adaptive equalizing mode is carried out. The adaptive
equalizing mode is composed of a former stage which employs the
training signals and a latter stage which employs the data (see
Table 4). In the adaptive equalizing mode, the digital modulator 24
is inactive, and the training signals or the data read from the
buffer memory 24 is supplied directly to the filter unit 11. In the
adaptive equalizing mode, the equalization error calculator/memory
26 and the initial value setting unit 30 are also inactive. The
carrier offset estimating unit 20D does not output the unit vector
(1+j0), but outputs a unit vector (described below).
In the former stage which employs the training signals, the carrier
offset estimating unit 20D outputs a unit vector having an initial
phase angle (-.theta.) which has been obtained in the carrier
offset estimating mode, to the complex multiplier 12. The complex
multiplier 12 cancels out a phase angle .theta. corresponding to
the carrier offset which has been estimated in the carrier offset
estimating mode. In the former stage, the coefficients of the
filter unit are updated using the training signals where the
estimated carrier offset is canceled out.
When the former stage is over, the selector switch 15 is shifted to
the terminal 152, the selector switch 27 is closed, and the data
are read from the buffer memory 23. At this time, the adaptive
equalizer according to the fourth embodiment operates in the same
manner as the adaptive equalizer according to the first embodiment
except that the buffer memory 23 is involved. The carrier offset
estimating unit 20D also operates in exactly the same manner as the
carrier offset estimating unit 20A according to the first
embodiment.
Since the adaptive equalizer according to the fourth embodiment
employs the training signals to estimate an initial value for a
carrier offset correcting quantity, it is possible to obtain
corrective data within a short period of time even when the carrier
offset is large.
Moreover, a carrier offset while a normal signal is being received
is corrected on the basis of a phase quantity based on the carrier
offset component, which phase quantity is extracted by accumulating
and adding distorted components in the transmission path over a
given period of time. Consequently, the carrier offset estimating
unit is relatively simple in arrangement.
Although certain preferred embodiments of the present invention has
been shown and described in detail, it should be understood that
various changes and modifications may be made therein without
departing from the scope of the appended claims.
* * * * *